The present invention relates to a nonvolatile memory, and more particularly relates to a nonvolatile semiconductor EE-PROM (Electrically Erasable-Programmable Read-Only Memory).
A programmable ROM is widely utilized in, for example, a computer-aided data processing system. Usually, a ROM can be programmed only once, either at the factory or in the field, by electrically fusing or otherwise removing unwanted internal links. Accordingly, the conventional programmable ROM has a defect in that a mistake in the programming cannot be corrected. For the purpose of overcoming the above mentioned defect, an EPROM (Erasable Programmable ROM) has been proposed and widely utilized in many nations. In the EPROM, the stored data can be erased by ultraviolet light and each bit can be electrically reprogrammed with appropriate voltage pulses. However, in the conventional EPROM, since ultraviolet light is required for reprogramming the stored data, it is not easy for an operator to freely reprogram the stored data.
In recent years, a nonvolatile EE-PROM has been proposed, for example, in the publication "Electronics," Feb. 28, 1980 pages 113 through 117, entitled "16-K EE-PROM Relies on Tunneling for Byte-Erasable Program Storage". The EE-PROM is basically comprised of a transistor having a source, a drain, a floating gate and a control gate, in which the floating gate partially includes an area of a very thin insulation layer on the order of a hundred A. Thus, a tunnel effect can be created, via the thin insulation, between the drain and the floating gate, and thereby data, to be stored in each memory cell, is reprogrammed.
The EE-PROM is very useful for an operator in, for example, the computer-aided data processing system, because, as previously mentioned, he can freely reprogram the stored data without using ultraviolet light. However, such an EE-PROM circuit of the prior art has the following two defects. Firstly as explained in detail below, undesired erasure of data often occurs in some of the half-selected memory cells. Secondly, it is not relatively easy to mass produce the EE-PROM circuit devices.